ABSTRACT
Space plasma is not limited to positive ions and electrons. There is a rapidly increasing number of indications that negative ions make up a significant component of planetary plasma environments. Although negative ions are known to play an important role in the Earth's ionosphere, no sensor designed to distinguish negative ions from electrons has yet flown beyond Earth. I present a brief overview of indicative negative ion observations in the Solar System, the potential importance of negative ions in planetary environments, and ongoing and future efforts to study these.